1. Field of the Invention
The present invention relates to signal processing, and more specifically, to signal processing techniques for gamma correction on an input video signal.
2. Description of Related Art
In order for a video display device to display a video signal, gamma correction is performed on the video signal so that the gamma characteristic of the video signal can match that of the video display device, since video display devices are of varying gamma characteristics according to the types of video display devices. For example, assuming that an available display is a Braun tube (CRT) display, the present television broadcasting is subjected, at the transmitting end, to the gamma correction for the gamma characteristic of CRT displays.
On the other hand, there has recently been an increasing variety of video display devices, and a liquid crystal display (LCD), a plasma display panel (PDP) and the like are also becoming widespread in addition to the CRT display. When a video display device other than the CRT display, such as the LCD or the PDP, displays a video signal for the present television broadcasting, it is required that the video signal be subjected to a process for undoing the gamma correction performed on the video signal at the transmitting end, and for matching the gamma characteristic of the video signal with that of the video display device in use.
The process for undoing the gamma correction performed previously on the video signal, which is also called “inverse gamma correction,” is essentially equivalent to the gamma correction in that the process is to change the gamma characteristic of the video signal. The process for changing the gamma characteristic of the video signal is herein referred to generally as “gamma correction” regardless of whether it is the gamma correction or the inverse gamma correction, unless otherwise specified.
The gamma correction is performed based on a gamma correction curve. For example, a lookup table (LUT) is created by including description of conversion data based on the gamma correction curve, and prestored in a storage device such as a ROM (read only memory). Then, the gamma correction is performed by using this LUT.
However, the high accuracy of approximation to the gamma correction curve is required so that a video can be displayed as clear and natural as possible. This requires such an enormous amount of data in the LUT that the storage device that stores the LUT should have a large capacity and that a gamma correction device should also have a large circuit scale. In particular, if it is necessary to support multiple video display devices of different gamma characteristics, the same number of LUTs as the number of types of video display devices is required, so that this problem becomes more noticeable.
Japanese Patent Laid open application No. 2004-140702 discloses an approach for suppressing the circuit scale increase of the gamma correction device. This approach involves: first setting multiple sample points by equally dividing a video signal between its minimum and maximum signal levels; and holding a yet-to-be-corrected signal level and a corrected signal level (sample data) in association with each other on each of the sample points. Assuming that the yet-to-be-corrected signal level and the corrected signal level are represented by values on the X axis and Y axis, respectively, the points indicated by the coordinates (the signal levels at the sample points, and the sample data) are located on the gamma correction curve. This approach uses the data previously held in this manner for cubic interpolation correction to thereby obtain the corrected signal level corresponding to the signal level of the input video signal.
FIG. 7 shows structural components, shown as functions in FIG. 5 of the Patent Literature, indicated as devices for sake of simplicity. As shown in FIG. 7, a gamma correction device 11 includes a sample data register 21 that stores sample data, a sample data selector 22, a kernel coefficient memory 23, a coefficient selector 24, and an interpolation operation unit 25.
The interpolation operation unit 25 performs cubic interpolation operation based on a third-order polynomial equation called “a kernel function”, and uses sample data fed from the sample data selector 22 and coefficients (namely, kernel coefficients) fed from the coefficient selector 24 in order to perform the interpolation operation.
The sample data selector 22 selects sample data required for the interpolation operation according to the signal level of an input video signal, and supplies the sample data to the interpolation operation unit 25. In the example of the gamma correction device 11 shown in FIG. 7, sample data at each two sample points on the right and left sides of the level of the input video signal on the X axis are selected.
The coefficient selector 24 obtains distances (level differences) between the level of the input video signal and sample points of four sample data selected by the sample data selector 22, reads out kernel coefficients for the obtained level differences from the kernel coefficient memory 23 and outputs the kernel coefficients to the interpolation operation unit 25.
The interpolation operation unit 25 performs weighting addition on the four sample data from the sample data selector 22 while using as weighting coefficients the four kernel coefficients from the coefficient selector 24, to thereby obtain the signal level of the gamma-corrected input video signal.
This approach enables the gamma correction using a configuration with a small-scale circuit or with a small amount of operation instead of using a large-scale LUT or performing higher-order operation.